I am giving information about these spiritual creatures, as someone asked me to a while ago i have been really busy. Sorry it took a while to get up.
Also as a "to let you know" You cannot physically shift into a hollowgast, you can only mentally and/or astrally. That goes for any shifting of any sort. I am saying this now, please respect that I'm saying this not to disapoint you but to educate you.
A Hollowgast:
A hollowgast is a spiritual creature/monster if you want to call them either, that feeds off of soul energy (like a psionic vampire), carnivores, and favors the eyes of their prey as a main meal. They are described of having pitch black eyes, as a description in a novel known as " Hollow city" They look like human corpses, said to have long claw like hands, massive jaws with 3-6 tentacle like tongues, etc.. the trait description could go on forever. They appear in a few different books, legends, and verbal stories. Hollows have different names for different stories, and legends based on the language that it was told in.
The origins of the legend:
Now, this is a speculation on the real origin as, the earliest known story was a verbal passed down by families. The time of creation of the legend came from early Whales area. The belief is that every new moon, harvest moon, full moon, and eclipse the villagers would have to sacrifice something they loved or livestock, there has been sightings of the creature. Ranging from northern whales, around the shoreline of the UK, parts of Poland, switzerland, and surprisingly parts in Africa.
Why it's such a unknown legend:
Like i have stated above- The hollowgast goes by many different names and titles, depending on which language it was spoken or told in.
Below i will give you a link to a spell or i will call it an provoking chant, that helps aid in mental and astral shifting. The spell says it's for physically shifting, but like i said that it's not possible. If you have a question about the topic, you can message me if i didn't cover it in here. Thank you for reading.
There is also people stating you cannot mentally shift, there is a practice called shaminisim that is basically mentally shifting. I don't know that i spelled it right, nor have i practiced that art.